Clamp Pressure: The final word

After our discussion here, I wrote to technical support at Titebond
regarding our discussion of clamp pressure. I got a next day reply from a
very knowledgeable and helpful gentleman, Mr. Zimmerman. I'm posting it
here.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I am writing in response to your question about clamp pressure. First,
your calculation and understanding is correct. If you wanted to produce
200 psi over an area 12" x 12", you would need 28,800 pounds of force. On
the other hand, it is not clear whether you often, or ever, fell short of
the actual, required clamp pressure.
The actual required clamp pressure for any bond involving a wood glue is a
combination of the small amount of pressure required to squeeze the glue
into a thin, consistent layer, and the pressure necessary the compensate
for any distortion or lack of fit in the wood stock being used. That means
when the surfaces of the pieces being joined are true, and there is no gap
between the pieces when they are dry fit, very little pressure is required.
If, however, the same assembly is being made using pieces which are bowed,
twisted or ill-fitted, the required pressure is much greater, and is
largely the pressure required to straighten the wood and pull it into
position. Thus, the actual required pressure for a bond also reflects the
thickness, or fight, of the wood involved, with much more pressure
obviously required to straighten a very thick piece of maple or oak than to
straighten a thinner piece of the same species.
In many applications, then, pressure, serves to compensate for some lack of
diligence in wood preparation. That being the case, good wood preparation
lessens the need for, or dependence on, pressure. In the case of our
literature, the high suggested pressures reflect the fact that those
individuals being addressed include those who, at least on occasion, are
trying to bond thick, poorly fitted pieces of wood, and for those readers,
the high, suggested values are, indeed, necessary.
Finally, because the bond strength produced in a joint is the result of the
entanglement of the glue particles which have been drawn into the pores and
anchored to the wood on the two sides of the joint, there is rarely any
concern for applying so much pressure that the glue is all squeezed out.
In fact, the bond strength achieved increases as the bondline or layer of
glue becomes thinner. Given that fact, there are only two situations in
which high pressures may be counterproductive. First, there is always a
concern that the pores of wood at the bonding surfaces not be crushed, and
that is the reason that our listed pressures are lower for the softer
woods. The second situation deals with bonds involving end grain or other
open grain. There the concern is that the open grain is prone to suck up a
large amount of glue and, if that thirst has not been quenched before
clamping, that excessive absorption of glue may result in a starved, and
weak joint. Because most bonds involve face or edge grain which is
relatively straight, that particular risk is rarely a concern.
I hope this response is helpful, and ask that you feel free to write again
or to call me at 1-800-###-#### if I can be of any further assistance.

<snip great bit of information>
This only serves to confirm my feelings in this area. Graham Blackburn
teaches an annual joining class. In that class, once the students have
planed a good joint, they are instructed to apply a minimal amount of glue
to one board edge, then rub the edge of the other board onto the first,
squaring everything up by finger feel. No clamps are used. The joined
boards are simply placed against a wall to dry while the class goes to
lunch. At the end of the day, they are instructed to break the joint. The
joint always fails in the wood, not in the glue.
Cheers,
Eric

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